Disclosed herein is a phase change ink composition including a synergist for pigment dispersion. More particularly disclosed is a phase change ink composition comprising a pigment; a carrier; a dispersant; and a synergist, (a) wherein, for a given sample of the synergist, about 80 to about 100 percent of the sample has a volume average particle diameter of about 50 to about 200 nanometers; (b) wherein the synergist has a surface area of about 30 to about 100 m2/gram; and (c) wherein the synergist has a density of above about 0.6 to about 2 grams/cm3.
In general, phase change inks (sometimes referred to as solid inks or “hot melt inks”) are in the solid phase at ambient temperature, but exist in the liquid phase at the elevated operating temperature of an ink jet printing device. At the jet operating temperature, droplets of liquid ink are ejected from the printing device and, when the ink droplets contact the surface of the recording substrate, either directly or via an intermediate heated transfer belt or drum, they quickly solidify to form a predetermined pattern of solidified ink drops. Phase change inks have also been used in other printing technologies, such as gravure printing.
Phase change inks for color printing typically comprise a phase change ink carrier composition which is combined with a phase change ink compatible colorant. In a specific embodiment, a series of colored phase change inks can be formed by combining ink carrier compositions with compatible subtractive primary colorants. The subtractive primary colored phase change inks can comprise four component dyes, namely, cyan, magenta, yellow and black, although the inks are not limited to these four colors. These subtractive primary colored inks can be formed by using a single dye or a mixture of dyes. For example, magenta can be obtained by using a mixture of Solvent Red Dyes or a composite black can be obtained by mixing several dyes. U.S. Pat. No. 4,889,560, U.S. Pat. No. 4,889,761, and U.S. Pat. No. 5,372,852, the disclosures of each of which are totally incorporated herein by reference, teach that the subtractive primary colorants employed can comprise dyes from the classes of Color Index (C.I.) Solvent Dyes, Disperse Dyes, modified Acid and Direct Dyes, and Basic Dyes.
The colorants can also include pigments, as disclosed in, for example, U.S. Pat. No. 5,221,335, the disclosure of which is totally incorporated herein by reference.
Phase change inks are desirable for ink jet printers because they remain in a solid phase at room temperature during shipping, long term storage, and the like. In addition, the problems associated with nozzle clogging as a result of ink evaporation with liquid ink jet inks are largely eliminated, thereby improving the reliability of the ink jet printing. Further, in phase change ink jet printers wherein the ink droplets are applied directly onto the final recording substrate (for example, paper, transparency material, and the like), the droplets solidify immediately upon contact with the substrate, so that migration of ink along the printing medium is prevented and dot quality is improved.
Compositions suitable for use as phase change ink carrier compositions are known. Some representative examples of references disclosing such materials include U.S. Pat. No. 3,653,932, U.S. Pat. No. 4,390,369, U.S. Pat. No. 4,484,948, U.S. Pat. No. 4,684,956, U.S. Pat. No. 4,851,045, U.S. Pat. No. 4,889,560, U.S. Pat. No. 5,006,170, U.S. Pat. No. 5,151,120, U.S. Pat. No. 5,372,852, U.S. Pat. No. 5,496,879, European Patent Publication 0187352, European Patent Publication 0206286, German Patent Publication DE 4205636AL, German Patent Publication DE 4205713AL, and PCT Patent Application WO 94/04619, the disclosures of each of which are totally incorporated herein by reference. Suitable carrier materials can include paraffins, microcrystalline waxes, polyethylene waxes, ester waxes, fatty acids and other waxy materials, fatty amide containing materials, sulfonamide materials, resinous materials made from different natural sources (tall oil rosins and rosin esters, for example), and many synthetic resins, oligomers, polymers, and copolymers.
Ink jetting devices are known in the art, and thus extensive description of such devices is not required herein. As described in U.S. Pat. No. 6,547,380, which is hereby incorporated herein by reference in its entirety, ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field that adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
There are at least three types of drop-on-demand ink jet systems. One type of drop-on-demand system is a piezoelectric device that has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. Another type of drop-on-demand system is known as acoustic ink printing. As is known, an acoustic beam exerts a radiation pressure against objects upon which it impinges. Thus, when an acoustic beam impinges on a free surface (i.e., liquid/air interface) of a pool of liquid from beneath, the radiation pressure which it exerts against the surface of the pool may reach a sufficiently high level to release individual droplets of liquid from the pool, despite the restraining force of surface tension. Focusing the beam on or near the surface of the pool intensifies the radiation pressure it exerts for a given amount of input power. Still another type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink vehicle (usually water) in the immediate vicinity to vaporize almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands.
In a typical design of a piezoelectric ink jet device utilizing phase change inks printing directly on a substrate or on an intermediate transfer member, such as the one described in U.S. Pat. No. 5,372,852, which is hereby incorporated herein by reference in its entirety, the image is applied by jetting appropriately colored inks during four to eighteen rotations (incremental movements) of a substrate (an image receiving member or intermediate transfer member) with respect to the ink jetting head, i.e., there is a small translation of the print head with respect to the substrate in between each rotation. This approach simplifies the print head design, and the small movements ensure good droplet registration. At the jet operating temperature, droplets of liquid ink are ejected from the printing device and, when the ink droplets contact the surface of the recording substrate, either directly or via an intermediate heated transfer belt or drum, they quickly solidify to form a predetermined pattern of solidified ink drops.
Thermal ink jet processes are well known and are described, for example, in U.S. Pat. Nos. 4,601,777, 4,251,824, 4,410,899, 4,412,224 and 4,532,530, the disclosures of each of which are hereby incorporated herein.
As noted, ink jet printing processes may employ inks that are solid at room temperature and liquid at elevated temperatures. For example, U.S. Pat. No. 4,490,731, which is hereby incorporated by reference herein, discloses an apparatus for dispensing solid ink for printing on a substrate such as paper. In thermal ink jet printing processes employing hot melt inks, the solid ink is melted by the heater in the printing apparatus and utilized (i.e., jetted) as a liquid in a manner similar to that of conventional thermal ink jet printing. Upon contact with the printing substrate, the molten ink solidifies rapidly, enabling the colorant to substantially remain on the surface of the substrate instead of being carried into the substrate (for example, paper) by capillary action, thereby enabling higher print density than is generally obtained with liquid inks. Advantages of a phase change ink in ink jet printing are thus elimination of potential spillage of the ink during handling, a wide range of print density and quality, minimal paper cockle or distortion, and enablement of indefinite periods of nonprinting without the danger of nozzle clogging, even without capping the nozzles.
Examples of the phase change inks herein are inks that include an ink vehicle that is solid at temperatures of about 23° C. to about 27° C., for example room temperature, and specifically are solid at temperatures below about 60° C. However, the inks change phase upon heating, and are in a molten state at jetting temperatures. Thus, the inks have a viscosity of from about 1 to about 20 centipoise (cp), for example from about 5 to about 15 cp or from about 8 to about 12 cp, at an elevated temperature suitable for ink jet printing, for example temperatures of from about 60° C. to about 150° C.
In this regard, the inks herein may be either low energy inks or high energy inks. Low energy inks are solid at a temperature below about 40° C. and have a viscosity of from about 1 to about 20 centipoise such as from about 5 to about 15 centipoise, for example from about 8 to about 12 cp, at a jetting temperature of from about 60° C. to about 100° C. such as about 80° C. to about 100° C., for example from about 90° C. to about 100° C. High energy inks are solid at a temperature below 40° C. and have a viscosity of from about 5 to about 15 centipoise at a jetting temperature of from about 100° C. to about 180° C., for example from 120° C. to about 160° C. or from about 125° C. to about 150° C.
While certain colorants suitable for use in phase change inks are known, an increase in the range of colorants suitable for use in phase change inks is desirable. Current phase change inks often comprise custom dye colorants. These custom dyes are very expensive. It is desired to replace custom dye colorants with less expensive colorants. Pigments are typically much less expensive than dye colorants. In addition, pigments can offer improved colorfastness over dyes, reduce or eliminate migration issues, and improve ink robustness characteristics.
Pigmented phase change ink compositions that include various dispersants are also known. However, the use of certain polymeric dispersants can be disfavored in some phase change inks for a variety of reasons. The problems caused by the use of certain polymeric dispersants include a negative effect on rheological properties of the ink, such as non-Newtonian behavior and an increase in viscosity.
Pigment particles in the ink must be properly dispersed and stabilized such that the ink can be reliably jetted without the clogging of the print heads by the pigment particles. Polymeric dispersants in phase change inks can affect drop formation, because polymers will tend to form filaments which affect the formation of small drop sizes. Most of the commercially available dispersants were designed for aqueous based and solvent based ink systems and are not compatible with hydrophobic wax based inks. Many of the commercially available compounds that can effect dispersion of pigments in low polarity inks (usually solvent-based) are liquids or pastes and are not designed to chemically withstand the excessive temperatures in the printer (over 100° C., typically 115° C. to 120° C.) for long periods of time. Furthermore, the use of polymers in solid ink can be disfavored for the following reasons: a) they can have a negative impact on rheological properties producing non-Newtonian behavior and an increase in viscosity, and b) they can affect drop formation during jetting, polymers will tend to form filaments which might affect the formation of small drop sizes.
Dispersion of pigments in solid ink formulations can be particularly difficult owing to the high temperatures incurred in the printer (such as greater than 100° C.), the potential for multiple freeze-thaw cycles the ink might undergo, and the non-polar nature of the ink vehicle.
U.S. Pat. No. 7,780,774, of Raymond W. Wong, et al., which is hereby incorporated by reference herein in its entirety, describes in the Abstract thereof a method of making a phase change ink composition includes adding to a device with a stirring element at least one pigment, at least one dispersant and at least one synergist, processing the device at an elevated temperature and at a stirring rate to wet the at least one pigment to obtain an ink concentrate, and mixing the ink concentrate with an ink vehicle to obtain the ink composition. The device may be an extruder, an attritor or the like. The method allows for the reduction in the amount of dispersant to include in the ink.
While known compositions and processes are suitable for their intended purposes, a need remains for improved phase change ink compositions. There is further a need for an improved colored phase change ink composition where the pigment particles are stable and well dispersed in the ink. There is further a need for phase change ink compositions including improved synergists that are chemically stable, compatible with the phase change ink formulation and that can provide stabilization of pigment particles in phase change inks over long periods of time at high temperatures. There remains a need for pigmented phase change inks with improved image quality, improved light fastness, and reduced show through. A need also remains for pigmented phase change inks where the colorants have reduced agglomeration and settling in the ink when the ink is exposed to high temperatures for prolonged periods. A need also remains for pigmented phase change inks with reduced clogging of the jets in the print head. There further remains a need for pigmented phase change ink compositions where dispersion of pigments in the solid ink compositions is stable under high temperature conditions incurred in the printer (such as greater than 100° C.), that is stable throughout multiple freeze-thaw cycles the ink can undergo, and that is compatible and stable in the non-polar ink vehicle.
The appropriate components and process aspects of the each of the foregoing U.S. Patents and Patent Publications may be selected for the present disclosure in embodiments thereof. Further, throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.